1. The problem statement, all variables and given/known data I'm just confused by notation in this question, because the textbook I use has different notation than my course notes and now I'm just not sure what anything means. 2. Relevant equations Group velocity x Phase velocity = c^2 3. The attempt at a solution What is c in this case? Is it the speed of light, or some other velocity? I know it's a really stupid question... Ta for any help though.